1. Field of the Invention:
An orifice-type pressure reduction device compatible with particle sampling from compressed, high-purity gases is described.
2. Discussion of the Background:
Pressure reduction is a necessary step in sampling particles from compressed gases because most particle counters operate at or near atmospheric pressure. The criteria for a good pressure reducing device are that the particle loss is negligible and no artifact particles are generated.
It has been discovered from past experience that pressure regulators, needle valves and other commonly used flow control devices cannot be used for particle sampling because particles are lost in an uncontrolled fashion.
Pressure reduction by expanding gases through a capillary type pressure reducer tube for sampling from pressurized gas line has been used, for example in the high pressure diffuser shown in FIG. 1. FIG. 1 discloses a diffuser 1, needle assembly 2, end cap 3, jet guard 4, diffuser mount 5 and case member 6. An inlet jet 7, adapter fitting 8, hex cell 9 and high pressure gas source 10 are also utilized. This method and device has a severe drawback with respect to cleanability which thus prevents its application in high purity gases. Previous data ("Particles in Ultrasonic Process Gases", G. Kasper and Y. Wen Handbook of Contamination Control in Microelectronics Industry, Jan., 1988) have shown that the device itself is an indefinite source of particles.
Expanding gases through a thin orifice disc was recognized as the most suitable method of reducing pressure for particle analysis (Wen and Kasper, 1988, Journal of Aerosol Research, Japan; 1988 SEMI International Standards). The fluid dynamics for flow passing through an orifice has been well documented.